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Did a Cosmic Impact Kill the Mammoths?

The Impact Hypothesis

The rise and
fall of the theory that cosmic catastrophes altered human prehistory
in North America.

Ever since
the Alvarez (1980) hypothesis that the end-Cretaceous (Cretaceous-Tertiary
or KT) mass extinction was the result of a cosmic impact sixty-five
million years ago, the idea of killer asteroids or comets has been frequently
discussed. The stunning confirmation of the KT impact initiated a revolution
in our thinking about possible external events and their effects on
biological evolution. David Raup of the University of Chicago famously
proposed that perhaps all major mass extinctions were impact induced.
He even published a "kill curve," suggesting that lesser extinctions
might be the result of smaller impacts. Unfortunately for those of us
who sought a general explanation for mass extinctions, these broader
suggestions have not been verified. It seems increasingly likely that
cosmic impacts are only one of several catastrophic events that have
produced mass extinctions. Still, the discovery that an impact sixty-five
million years ago led to the extinction of the dinosaurs remains one
of the iconic ideas of late twentieth century science.

The
most dramatic recent hypothesis linking extinctions with impacts was
proposed in 2007 by a team of twenty-six scientists, led by nuclear
chemist Richard Firestone of Lawrence Berkeley National Laboratory,
with independent geophysicist Allen West; geologist James Kennett of
the University of California, Santa Barbara (a member of the National
Academy of Sciences); and archaeologists Douglas Kennett and Jon Erlandson
of the University of Oregon. In a widely reported presentation at
a joint assembly of the American Geophysical Union (AGU) in Acapulco,
Mexico—followed a few months later by a paper in the Proceedings
of the National Academy of Sciences (PNAS)—these
scientists proposed a cosmic origin for a geologically recent event,
the extinction of many large mammals (megafauna) in North America approximately
13,000 years ago. The events they linked were the presence of a dark
soil layer that coincided with the extinction of megafauna (including
the mammoth and mastodon), the end of the Clovis culture (identified
by its large and well-made spear points), and the start of the Younger
Dryas (YD) cool period (a millennium pause in the general warming at
the end of the last ice age). The in
situ bones of extinct
megafauna, along with Clovis stone tools, occur below this black mat
but not within or above it. At this boundary the team reported finding
enriched levels of iridium and other signatures of extraterrestrial
material.

In
a sweeping conclusion reminiscent of the Alvarez hypothesis, Firestone
and his colleagues postulated that these events were tied to one or
more cosmic impacts over North America, releasing energy they estimated
at about ten million megatons (equivalent to an impacting comet four
kilometers in diameter). They suggested that an airburst and/or surface
impact by a dense swarm of carbonaceous asteroids or comets set vast
areas of the North American continent on fire. This swarm would have
exploded above or even into the Laurentide Ice Sheet north of the
Great Lakes. Such an airburst would have been a million times larger
than the Tunguska impact event of 1908.

Scientific Reactions

While archaeologists
pondered the reality of this sharp boundary layer and the new evidence
of extraterrestrial materials, a few astronomers and impact experts
immediately questioned this scenario. They noted that there was no mechanism
to hold such a dense swarm of impactors together in space. To the suggestion
that a large comet had broken up just before hitting Earth, they replied
that this lacked a physical mechanism. If the comet had shattered when
it encountered the atmosphere at an altitude of about one hundred kilometers,
the lateral dispersion would be at most tens of kilometers, hardly enough
to distribute the effects across North America. An alternate suggestion
was that this event was analogous to the 1992 tidal break-up of comet
Shoemaker-Levy 9, which resulted in the separate impact of about twenty-three
fragments on Jupiter two years later. However, these comet fragments
were spread over more than a million kilometers in space, and the impacts
were distributed over all longitudes on Jupiter. While it is true that
some comets have been seen to spontaneously disintegrate in space, the
chances of this happening just before an impact with Earth is negligible—something
that might have happened at most once in the past four billion years.
There was apparently no way to get a swarm of impactors to target North
America alone.

One
of Firestone and his colleagues' suggestions that troubled geologists
and impact experts was that the same event (or a similar one) might
have been responsible for the Carolina Bays geologic formation. The
Carolina Bays are several hundred thousand shallow, elliptical depressions
of disputed origin along the U.S. eastern seaboard. Firestone suggested
that each of these more than 100,000 features was the result of a cosmic
impact. Since the well-known Tunguska airburst in Siberia in 1908 did
not form a crater, the implication is that these were made by larger
objects that reached the ground. But calculation of average impact frequency
suggested that only about one super-Tunguska could be expected to hit
Earth in the past 13,000 years. The chances of two such extremely unlikely
swarm impacts happening within the past few thousand years is worse
than negligible.

A
warning of the problems with this hypothesis should have been apparent
to anyone who read 2006's The
Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History
of Civilization by Richard
Firestone and Allen West, with writer and publicist Simon Warwick-Smith.
This trade book, which appeared a year before Firestone's AGU presentation,
described the YD impact hypothesis as part of a much larger cycle of
cosmic events. This book develops Firestone's 2001 suggestion that
a cosmic ray catastrophe, probably caused by a supernova, occurred in
northeastern North America in the late Pleistocene. He concluded that
massive thermal neutron irradiation radically altered the radioactivity
of terrestrial materials and "probably figured in the mass extinction
of Ice Age fauna." In The
Cycle of Cosmic Catastrophes,
Firestone links the YD impact to this postulated nearby supernova,
which he asserted took place 41,000 years ago and initially devastated
most life in Asia. Then 34,000 years ago the shock wave from this supernova
initiated another wave of intense cosmic bombardment of Earth. The only
evidence for this event is the remarkable claim that mastodon tusks
from about that time are pitted with cosmic dust, suggesting that these
animals received the direct blast of supernova material striking Earth
(unstopped, apparently, by our atmosphere).

In The Cycle of Cosmic Catastrophes, the debris cloud from the supernova
is supposed to have reached Earth about 13,000 years ago. The YD impact
was one manifestation of this blast wave, bathing the planet in radioactivity
and destabilizing the magnetic field. In this book the authors suggest
that the Carolina Bays were created by secondary impacts of ejecta from
the main hit in the North American ice sheet. To produce this much ejecta,
the hit must have been among the most catastrophic events in Earth's
history. They suggested that the YD impact excavated Hudson Bay, making
it larger than the KT impact of sixty-five million years ago, which
is estimated to be a once-in-one-hundred-million-years event. Yet supposedly
this huge hit did not produce a worldwide mass extinction but influenced
only the megafauna of North America. This entire scenario is inconsistent
with what astronomers know about supernovas, which Phil Plait summarized
in his recent book Death
from the Skies. It raises
serious questions about the reliability of the PNAS paper that Firestone and West,
with two dozen additional authors, published a year later.

New Data and Continued Controversy

In January
2009, Doug Kennett published a paper in Science asserting that nanodiamonds provide
the strongest evidence for the impact hypothesis, with multiple airbursts
and impacts at the onset of the YD cooling. He argued that these nanodiamonds
were produced in the moderate shocks associated with comet airbursts.
By this time, earlier claims about iridium enrichment and other possible
impact markers had been withdrawn. The usual geological evidence of
large crater-forming impacts such as the KT, namely shocked quartz,
had never been reported at the YD boundary sites. Now the nature and
origin of nanodiamonds became the primary issue.

There
were a variety of claims and counterclaims concerning the nanodiamonds.
Were they produced in the impact, or were they primordial material
trapped in the comet when it formed billions of years earlier? Most
impact experts agree that nanodiamonds were unlikely to have been formed
in the impact. In fact, Mark Boslough of Sandia National Laboratories
calculated that the high temperatures and pressures in a large impact
would likely destroy existing nanodiamonds. Some note that nanodiamonds
are actually ubiquitous on Earth and can even be formed in fires. One
scientist joked that perhaps the nanodiamonds were concentrated at
human habitation sites where hunters were roasting the meat from mammoths
and mastodons. The history of these claims and counterclaims is well
documented in articles by Science journalist Richard Kerr published
in 2007, 2008, and 2009.

At
a meeting of the Geological Society of America (GSA) in October 2009,
several presentations argued strongly against the YD impact from a variety of perspectives (see GSA summary in references). One paper claimed that
the black mats at the YD boundary were not charcoal from widespread
fires but rather peat-rich dark soils formed during a wet period. Another
speaker noted that there was no archeological evidence for a sudden
decline in the human population of North America at the YD. While one
speaks of the end of the Clovis culture, this only means that the style
of stone tools changed. We don't know why, although one possibility
is a shift to hunting smaller animals. Other scientific teams reported
that their efforts in the field to find nanodiamonds or other impact
markers at the YD boundary layer were unsuccessful.

Some
impact proponents who were not present at the GSA meeting wrote blogs
and circulated e-mails accusing these scientists of sloppy fieldwork.
They asserted that the boundary layer was very thin and rather spotty
in distribution, requiring care to find it—care they implied had not
been exercised by their critics. The GSA session resulted in the undercutting
of the credibility of the original PNAS and Science papers, but since the two sides
did not confront each other directly, nothing was settled.

The American Geophysical Union
Symposium

Given the
conflicting interpretations concerning a possible YD impact catastrophe,
many scientists thought a debate between proponents and critics might
help clear the air. The YD impact hypothesis had been discussed for
more than two years without any common ground emerging. Indeed, the
original team of twenty-six scientists was itself fragmenting, with
only Richard Firestone and Allen West still strongly advocating the
original multi-comet impact scenario. Mark Boslough of Sandia worked
with Allen West to organize a symposium at the 2009 fall meeting of
the AGU, with speakers from both sides. While no one expected that public
discussion would lead to reconciliation, the organizers hoped this symposium
would at least focus on the main issues.

The
December 2009 AGU session topic was "Younger Dryas Boundary: Extraterrestrial
Impact or Not?" Ten speakers were squeezed into a single two-hour
session, including Allen West, impact specialist Peter Schultz of Brown
University, and former NASA geoscientist Ted Bunch from among the original PNAS
authors. Firestone chose not to attend. There was standing room only
at the session, and several hundred others were turned away at the door.

The
star of the event was Wally Broecker of the Department of Earth and
Environmental Sciences at Columbia University. Broecker is one of
the most respected environmental scientists in the world. Credited with
first describing the ocean current conveyer belt and inventing the term
"global warming," his honors include membership in the National
Academy of Sciences and award of the Presidential Medal of Science.
His presentation was sober and low key, but he made it clear that he
was unconvinced by the evidence for an impact or any catastrophic change
at the YD boundary. But rather than condemning the hypothesis, he stated
simply that the decline in the North American megafauna could be understood
as a result of climate change and overhunting—the conventional explanation.
Broecker said, "We do not need the impact hypothesis."

Most
of the speakers who followed Broecker restated positions that were already
on the record. West and his colleagues repeated their evidence of extraterrestrial
markers in the black mat at the YD boundary, with emphasis on the presence
of nanodiamonds. They suggested several possible impact scenarios,
such as oblique impact on the ice sheet, but admitted that there were
many uncertainties. Several critics reiterated that the proposed impact
is highly unlikely statistically and that an airburst as large as proposed
is inconsistent with our understanding of comets and the impact process.

The
most interesting new results were presented by Jacquelyn Gill, a graduate
student in the Department of Geography at the University of Wisconsin–Madison.
She has been studying lake sediments that contain spores of sporomiella
(a fungus that occurs in herbivore dung) in the time range around the
YD. This fungus is related to the total mass of herbivores and can be
used as a proxy for the megafauna population. Her data show a gradual
decline, beginning well before the YD marker and extending beyond
the end of the Younger Dryas cool period. Indeed, in some isolated locations
mammoths and mastodons did not go extinct until much later: there were
dwarf wooly mammoths on Wrangle Island in Alaska until about four
thousand years ago. Some large North American mammals did not go extinct
at all, including the bison, the moose, and the grizzly bear. Gill's
results seem consistent with the worldwide evidence that rapid declines
in large mammal population accompanied the arrival of early human hunters,
presumably as a consequence of overhunting.

Unfortunately,
the overcrowded session ran late, and there was no time for discussion
or questions. Even when their conclusions were challenged, most of the
scientists in the audience chose not to respond. The result was a lost
opportunity for real debate. Perhaps not surprisingly, the AGU session
received very little press attention. Indeed, following the AGU and
GSA meetings, the YD impact hypothesis seems to have retreated into
the obscurity of a few e-mail list-serves and blogs, such as "The
Cosmic Tusk" where George Howard (one of the original PNAS
authors) is presiding over a variety of catastrophist interpretations
of Holocene history.

Conclusions

It is instructive
to compare the trajectories of the YD and KT impact hypotheses, as there
are close parallels. Both research teams were led by nuclear scientists
(Luis Alvarez and Richard Firestone) from the University of California,
Berkeley. Both challenged the orthodoxy of mass extinctions. Both postulated
an environmental catastrophe triggered by a large cosmic impact. Both
were published initially in prestigious journals (Science and PNAS). They each presented a grand synthesis,
not only identifying evidence of extraterrestrial materials at the
extinction boundary but also proposing a broad impact scenario to explain
a wide variety of previously unrelated data. And both ideas were initially
resisted by the "old guard" of paleontologists and archaeologists.

While
each hypothesis encountered initial resistance, the KT impact theory
also gained enthusiastic support (see popular accounts by Walter Alvarez
and James Powell). The first confirming paper was published within weeks,
and soon multiple impact markers had been identified at a number of
additional exposures of the KT boundary. Astronomers and geologists
praised the paper and provided context by estimating the impact rate
for ten-kilometer comets and asteroids. Atmospheric scientists such
as Brian Toon and Kevin Zahnle of NASA Ames Research Center calculated
the dispersion and lifetime of dust ejected into the stratosphere by
the impact. Paleontologists like Peter Ward (University of Washington)—who
initially argued for a gradual decline of populations—gathered new
field data and used modern statistics to support an abrupt extinction
at the KT boundary. Within three years the first of a series of Snowbird
Conferences was held, bringing together top scientists to discuss the
role of cosmic impacts on the evolution of life. The idea of an impact
extinction gained early and continuing currency in the press.

In
contrast, efforts by other scientists to confirm the presence of impact
markers at the YD boundary have so far been unsuccessful. Astronomers,
rather then welcoming the impact idea, have raised serious objections
to the proposal by Firestone and colleagues. New data on megafauna extinction,
such as the work of Gill, point to a gradual decline. Archaeologists
emphasize that changing styles in stone tools do not demonstrate a sudden
shift in human populations at the start of the YD but merely a change
in technology or hunting style. In the aftermath of the 2009 GSA and
AGU meetings, the press seems to have lost interest, and continuing
support for the YD impact comes mostly from blogs by catastrophists
who have long advocated cosmic intervention in human history.

Even
without considering the technical issues at stake, there are two clues
that something is amiss with the YD impact hypothesis. First is the
2006 book The
Cycle of Cosmic Catastrophes,
which formulates the YD hypothesis within the context of catastrophist
pseudoscience. If more scientists and science journalists had been aware
of this earlier publication when the YD hypothesis was first published
in PNAS, it might never have gained traction.
Second is the absence of confirming or supporting papers by scientists
who were not members of the original team. A good hypothesis naturally
accretes confirmation and gets better with time, as did the Alvarez
KT impact hypothesis. Firestone's work has not done so.

It
seems clear that the YD impact proponents were trying to follow in the
footsteps of the Alvarez team, discovering evidence of a sudden extinction
event and linking this to an extraterrestrial impact. However, the story
isn't working out that way, and the impact they propose seems to be
virtually impossible. One parallel that troubles me, however, is that
the reaction of the traditionalists—scientists who say that the megafauna
were in decline anyway and "we don't need an impact"—rather
closely echoes the reaction of many old-guard scientists to the KT impact
hypothesis. There also may be philosophical and political overtones
that influence the reception given any proposal that deals with early
human history. There is a long tradition of catastrophist ideas, going
back to the biblical flood and Plato's story of Atlantis. Philosophically,
many people prefer the idea that humans have not had much effect on
the planet, either 13,000 years ago or today—better to blame thunderbolts
from the gods than to accept responsibility for our stewardship of Earth.

Acknowledgments

I am grateful
to Mark Boslough, Clark Chapman, and Alan Harris for many stimulating
discussions of the YD impact hypothesis and especially for their insightful
and generous suggestions for improving this paper.

References

Alvarez,
W. 1997. T. Rex
and the Crater of Doom.
Princeton University Press.

Firestone,
R.B., and W. Topping. 2001. Terrestrial evidence of a nuclear catastrophe
in paleoindian times. Mammoth
Trumpet Magazine (March):
9, published by the Center for the Study of the First Americans.

Firestone,
R., A. West, and S. Warwick-Smith. 2006. The
Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History
of Civilization. Bear
and Company, Rochester, Vermont.

Firestone,
R.B., et al. 2007. Evidence for an extraterrestrial impact 12,900 years
ago that contributed to the megafaunal extinctions and the Younger Dryas
cooling. Proceedings
of the National Academy of Sciences
104: 1616.

Geological
Society of America (GSA) Annual Meeting (October 18–21, 2009), Portland,
Oregon. Relevant oral presentations, with quotes from their abstracts.
Paquay et al.: No evidence of extraterrestrial geochemical components
at the Bølling-Allerød/Younger Dryas transition. ("Our study discredits
the YD impact hypothesis.") Surovelle and Holliday: Non-reproducibility
of Younger Dryas extraterrestrial impact results. ("We were unable
to reproduce any results of the original Firestone et al. study and
find no support for Younger Dryas extraterrestrial impact.") Pinter
et al.: Extraterrestrial and terrestrial signatures at the onset of
the Younger Dryas. ("Many of the purportedly unique markers at the
YD boundary layer were found in most or all other sites and horizons
analyzed, often at concentrations much higher than at the YD layer itself.")
Holliday and Meltzer: Geoarchaeology of the 12.9 ka impact hypothesis.
("Sites purported to provide direct evidence of the 12.9 ka impact
are not well constrained to that time. An ET impact is an unnecessary
‘solution' for an archaeological problem that does not exist.")

David Morrison

David Morrison is a long-time NASA senior scientist and Committee for Skeptical Inquiry fellow. He now divides his time between the SETI Institute and the NASA Lunar Science Institute. He hosts the "Ask an Astrobiologist" column at NASA's website.

Content copyright CSI or the respective copyright holders. Do not redistribute without obtaining permission. Thanks to the ESO for the image of the Helix Nebula, also NASA, ESA and the Hubble Heritage Team for the image of NGC 3808B (ARP 87).